Contagious ecthyma, Biology

Contagious ecthymaThe contagious ecthyma virus is immunologically distinct from vaccinia, but similar to the causative agent of pseudo-cow-pox. It belongs to the genus Parapoxvirus in the family Poxviridae. It affects lambs and kids of over 2 years old. The virus withstands drying and is capable of surviving at room temperature for at least 15 years. The disease occurs during dry season. Recovered animals are solidly immune for 2 to 3 years, but no antibodies appear to be passed in the colostrum. Hence, newborn lambs of immune ewes are susceptible to this virus. In man, typical lesions occur at the site of infection. The lesions are very itchy and respond poorly to local treatment. The contagious ecthyma spreads rapidly in a flock through contact with affected animals or through inanimate objects.

Clinical signs: The formation of papules and vesicles on the skin of the lips and sometimes on the nostrils and eyes is seen. These readily change to pustules and finally heavy scabs appear. The affected lambs or kids can neither suckle nor graze, which leads to rapid emaciation. Healing is usually complete in 1 month. The scabs fall off by this time leaving the lips smooth and without scars. Systemic invasion occurs rarely and lesions may extend down the alimentary tract as well as the trachea. In rams, lesions are present on the scrotal sac.

Diagnosis: The lesions in the epithelium, viz. ballooning of cells leading to degeneration, vesicle formation and possible granuloma formations help in diagnosis. Inclusion bodies are absent. The isolation and identification of the virus in cell-cultures confirm the disease. The virus can be passaged in rabbits if large doses are placed on scarified skin or injected intradermally. Mild lesions develop on the chorio-allantois of 9-to 12-day-old chick embryos. Complement fixation and immunodiffusion tests are commonly used for diagnosis.Treatment, prevention and control: There is no specific treatment. Removal of the scabs and application of ointments or astringent lotions is practiced. The provision of soft and palatable food is recommended. In the early stages of outbreak, the affected animals should be isolated and the remainder animals vaccinated with 1% suspension of dried scabs in 50% glycerol saline by scarification on the inside of the thighs. Vaccinated animals develop a good grade of protection against the disease. Vaccination is of little value when a large number of animals are already affected. Persistence of the disease in a flock from year to year is common. The lambs should be vaccinated at 6 to 8 weeks of age. Vaccination is completely effective for at least 2 years, but the lambs should be inspected 1 week after vaccination to ensure that local reactions ('take') have resulted. The absence of a local reaction signifies lack of viability of the vaccine and consequent development of immunity. The immunity is not effective until3 weeks after vaccination.